Using the largest sample of boys with Attention Deficit/Hyperactivity Disorder (ADHD) and healthy male controls studied to date (n=57 and 55, respectively), we found statistically robust (p=.005) decreases in right prefrontal brain, right caudate, and globus pallidus. These differences support the hypothesis that ADHD reflects dysfunction of the prefrontal cortico-striato-cortical circuit, particularly on the right. Thirty girls who have ADHD (of a planned 60) have now been scanned along with 40 controls as well as 24 of a planned 30 boys who have ADHD and have not been previously treated with stimulants. We are also recruiting monozygotic twins who are discordant for ADHD. These structural magnetic resonance imaging (MRI) data represent a 'library' of brain images that can be reanalyzed with newer imaging techniques such as those required to quantify white and gray matter volumes. Parcellation or segmentation algorithms are now being applied to this "brain image bank" to derive more fine-grained information pertinent to anatomic and functional hypotheses of ADHD. The most direct transfer from our structural MRI findings is to our investigation of functional MRI (fMRI) in children with ADHD. FMRI studies with normal children show prefrontal activation with response inhibition tasks. FMRI studies of response inhibition in children with ADHD are underway.